Sound recording method and system



May 22, 1945. F. G. ALBIN SOUND RECORDING METHOD AND SYSTEM Filed Dec. 3l, 1942 2 Sheets-Sheet l MR. QW 6% s M W.. SJ M m 3 L A www $1. M SJW mm M w o3 um@ m @QM May 22, 1945. F. G. ALBIN 2,376,567

SOUND RECORDING METHOD AND SYSTEM Filed Dec. 3l, 1942 2 Sheets-Sheet 2 Apg-@rues O /if j/"EDEe/c@ 6. ALE/N,

1N VEN TO R ATTORNEY.

Patented May 22, 1945 SOUND RECORDING METHOD AND SYSTEM Frederick George Albin, Los Angeles, Calif., as-

signor to Radio Corporation of America, a corporation of Delaware Application December 31, 1942, Serial No. 470,886

Claims. ("Cl. P19-400.3)

. herewith, the manner of its organizationland the This invention relates to sound recording apparatus, and particularly to a system in which the light source itself is the modulating element.

There are several types of sound recording systems known in the art, some of which employ constant light sources wherein the light beam is vibrated by mechanical elements such as light valves or galvanometer mirrors, and others ofwhich employ variable intensity vapor or glow lamps wherein the intensity thereof is varied in accordance with the sound waves being recorded. 'I'he present invention is directed to a system of the `atter type wherein the light may be modulated by the displacement of the light, emitting element as well as by varying its intensity.

The system therefore has many'of the advan' tages ofthe glow lamp combined with the advantages of the systems using constant, intensity light sources. The system is also adaptable for recording a combination record wherein the average intensity is varied for noise reduction or volume control purposes, and the instantaneous values are varied for recording the sound signal itself, as is disclosed in Kreuzer Patent No. 2,357,- 665 of September 5, 1944.

Some of the advantages of the present system are the elimination of mechanical motion which avoids mechanical resonance and transient effects. Due to the lack of inertia, the distortion is low at all frequencies .and levels, while an produces a magnetic flux at right angles to the abundance of light lis provided. The system is arc stream I6. Variations in the magnetic flux rugged and safe from overload damage, is rela-` .in which the arc is located, caused by varying tively inexpensive and can produce a squeeze currents in the winding I5, willproduce amovetrack eiect, Y ment or displacement of the arc stream I6 from The principal object of the invention, there- :i5 top to bottom of the drawing. The two pole fore, is to' improveV on present methods of and pieces I I and I2 are so arranged as to form an means for sound recording. aperture through which light from the arc stream Another object of the invention is to provide an passes for use in recording on a film 25. improved light modulated sound recording sys- 'I'he light passing the yoke aperture is projected tem, bya lens 2B and a lens 2I, to a-slit mask 22 hav- A further object oi the invention is to provide ing a slit 23 therein. The light passing the slit 23 a sound recording system wherein the light emit- 'is' projected to the iilm 25 by objective lenses 26. ting element is displaced in accordance with the after passing the partially silvered or dichroic sound waves being recorded. p reflector 21, such a reflector being disclosed in A further object of the invention is to provide CfI-llins Patent NO- 2,274,530 0f February 24. 1942. a sound recording system wherein the light source The slit 2,3 is imaged by the lenses 28 on the lm may 'be simultaneously modulated in intensity 2E, thus providing a sharp image thereon. The and displacement. shaded portion between slit 23 and illm 25 repre- A further object of the invention is to provide a sents an absence of light over approximately oneglow lamp type. of recording system wherein light half of the image ofthe slit 23 on the illm when modulation is obtained by magnetically varying the arc stream I6 is in the position shown` in the position of the light source. Fig. l.

Although the novel features which are believed 'Ihe remainder of the recording system shown to be characteristic of this invention are pointed in Fig. 1 includes a microphone 29 feeding a pre- 53 ampliiier 3U which is connected to a power amout with particularity in the claims appended mode o1' its operation will be betterunderstood by referringl to the following description, read in conjunction with the accompanying drawings, in which:

Fig. i is a diagrammatic view oi a sound recording system embodying the invention.

Fig. 2 is a detailed cross-sectional view of the light source used in theA invention.

Fig. 3 is a graph showing the variation between the light intensity at diierent distances from the center line of the arc stream.

Figs. 4 and 5 are detailed views of different types of light slits used in the invention.

Fig. 6 is a detailed view of a modincation of the optical portion of the invention, and

F18. 7 is a detailed view of another method of modulating the light source in accordance with the invention.

Referring now to: Fig. 1, a capillary mercury vapor lampis shown at 5 which may be any wellknown type. such as an Hanovia AH-8. In Fig. 2, the'lamp 5 is shown with transparent wall section 6 and terminal electrodes I and 8 between which 25 the arc stream Iii-is produced. Surrounding the lamp is a. magnetic yoke Ill having two similar polarized poles at II yand I2, and a pole I3 of opposite polarity. This magnetic yoke is polarized by current; passing through a. winding I5 which e BingerPatent No. 2,292,166 of August 4, 1942;

Balsley Patent No. 2,242,638 of May 20, 1941,' or in copending applications, Ser. No. 470,683, led December 30, 1942: Ser. No. 467,442, filed November 30, 1942; Ser. No. 485,255, illed April 30, 1943; and Ser. No. 485,256, iiled April 30, 1943.

To produce a variable density sound record on I the nlm 2l, the arc stream Il is moved transversely across the aperture between the poles II and l2 in accordance with the amplitude and frequency of the signal being recorded. The image on the illm may be either solely a variable intensity image of constant width as shown in the arrangement of Fig. 1, or a variable width image in accordance with the shape ofthe slit masks, itbeing understood that a push-pull system may also be provided, in accordance with my co-pending appllcatiom Serial No. 447,797, tiled June 20, 1942. 'I'he arc stream may also be displaced and modulated electrostatically by using charged plates on opposite sides of the arc as is well known in cathode ray tube practice.

The slit 23 or the aperture between poles i I and l2, may be either large or small as compared lwith the arc stream width, Fig. 3 showing two slit widths superimposed upon a curve showing the intensity oi the arc stream at varying distances from the center line of the arc. For instance, at zero distance, the arc is considered to have a aavc, sev

theseilgureaitwillbereadllyseenthatany movement o! the arc across the apertures "A" and "B" will vary the light quanta passing therethrough, it being only necessary to displace the arc stream It in the neighborhood oi 50 mils with aperture "A" and in the neighborhood of 100 mils with aperture B to vary the iight between its maximum and minimum intensities, the range being greater for aperture B."

A simplified method of and means for light impressing the illm with variations in light quanta by arc displacements is illustrated in Fig. 6 wherein an aperture ll is provided in a mask Il located between the are stream I8 and the aperture formed between the pole pieces Il and.|2. The aperture 4I is focussed on a nlm Il by the lenses land Il. This arrangement eliminates several ot the optical elements of the system of Fig.' l and increases the illumination eiiiciency but provides an image which is somewhat less sharp than that from the system of Fig. l. With the arc It in the position shown the nlm image is illuminated at approximately one-half its maximum intensity which the shaded lines are supposed to illustrate.

In Fig. '7 a modiilcatiun is illustrated wherein a ilxed potential is applied across windings Il from .a suitable source or supply, such as a battery 4l,

thusproviding a constant magnetic field for the arc. A similar constant iield may be supplied by a permanent magnet as is done in loud speaker design. The arc in Fig. 'I is displaced or modulated by varying the current ow through the arc 100% intensity, the intensity decreasing equally on both sides thereof as the distance from its center line increases, the arc having an intensity of `approximately 5% at a distance of 100 mils. Now,

assuming that the aperture employed is of the width of aperture A or approximately 10 mils,

the amount of light reaching the illm may be considered as represented by the area between the dotted aperture lines and the solid intensity line. As the dotted lines are moved horizontally across the paper it will be noted that the included area will change, increasing toward the right and decreasing toward the left. Such a width aperture will produce a variable density record of high resolution and having a certain illumination eillciency. To increase the illumination efficiency an aperture of the width shown by aperture B" may be employed wherein the aperture is in the neighborhood of 100 mils wide, this width aperture giving a lower resolution than aperture A. Intermediate width apertures may also be employed, the narrower the aperture the higher the resolution and the lower the illumination eiciency for a given intensity arc. Y

To further illustrate the manner of modulation of the light with the two different sized apertures A and B, reference is made to Figs. 4 and 5 wherein may be observed the height ofthe apertures A and B with respect to the intensity gradient of the arc stream represented by the graduated shading. As the arc is dlspaced with respect to the slits or apertures, the eilect is as though the slits were moved across the arc. Thus. in this manner varying light cuantas reach the illmdn accordance with the arcs position. From stream, conductors l1 and 48 being connected to the electrodes 1 and 8 as shown in Fig. 2, and to the amplifier 3| ei Fig. 1 in series with an are power supply such as a battery I8. Used in this manner, the well known type of motor action is n obtained wherein the arc is displaced in accordance with the strength of the current in the arc and the strength of the magnetic iield. In this embodiment the inductive eect ci' the winding i 5 is not encountered in modulating the arc'while variations in light at the film caused by both displacement and arc intensity are obtained, thus providing increased sensitivity.

In addition to the individual use of the two types of modulations just described, that is, magnetic deilection of the arc stream by varying the magnetic ileld and deectlon of the arc stream by varying the current ilow therethrough, it is possible to simultaneously combine the two types of modulation during the recording of a sound signal. 'I 'hat is, the are may be displaced byvarying the current in the arc in accordance with the instantaneous values of the sound Waves be ing recorded as shown in Fig. 7, and the arc may be displaced by varying the strength of the magnetic eldin accordance with the average value of the sound waves being recorded. Thus, a form oi.' squeeze track noise reduction may be obtained in this way. For normal noise reduction action both the signal and noise reduction currents are impressed on the same modulating means. Also, the instantaneous signal variations may be impressed on the Winding I5 and the noise reduction currents impressed over conductors C1 and I8. 'I'his is also a marmer of obtaining amplitude control since one of the modulating means may be varied in accordance with level cues, either manually or automatically to provide a record such as shown in the above-identied application, Serial parte. which tact avoids resonance and other well known advantages of mechanical systems. The type of modulation is safe against overload -damage and is relatively inexpensive as compared with certain other types of mechanical modulators. Since the modulating element has a minimum oi inertia, low distortion at all frequencies and levels is obtained, while linearity is obtained through inverse feedback. Since a mercury vapor lamp is employed, an abundance of light is pro'- vided suitable for recording on fine grain film, particularly if large apertures such as aperture B of Figs. 3 and 5. are employed. Furthermore, a fundamental i'ault of gaseous lamp modulation, namely, gas ionization lag or inertia, is avoided. Arc displacement modulation also avoids a further fault of intensity modulation of gaseous lamps, particularly serious when noise reduction is employed, such as the change of ambient tem.- perature due to change of average lamp current, since with arc displacement modulation the average current, thence the ambient temperature remains constant at all times.

I claim as my invention:

l. A sound recording system comprising a gaseous light source, means for positioning said source in a magnetic field, an optical system for projecting light from said source to light sensitive material, and means for varying said magnetic eld in accordance with sound waves to be recorded.

said light source being displaced in accordance with said variations in magnetic field to vary the. light impressed on said light sensitive material.

2. A sound recording system comprising a substantially inertialess light source, means for positioning said source in a magnetic field, an optical system for projecting light from said source to light sensitive material, and means for varying said magnetic eld in accordance with sound waves to be recorded, said light source being displaced in accordance with said variations in magnetic field to vary the light impressed on said light sensitive material, the intensity of said light source being graduated away from its center, displacement of said source in one direction causing lower light intensities to be impressed on said light sensitive material and displacement of said source in the ,opposite direction causing higher light intensities tobe impressed on said material.

3. A sound recording system in accordance with claim l in which said light source is displaced in accordance with both the instantaneous values and the average value of said sound waves to be recorded. 4

4. A sound recording system comprising means for generating electrical currents corresponding to sound waves to be recorded, a gaseous light producing element, means for positioning said element in a magnetic eld of constant strength,-

an amplifier for said currents, means for connecting said ampliiier to said light producing element, a nlm and an optical system for p/roiecting light from said element to said film, variations of said electrical currents varying both the intensity and position of said element in said magnetic field to vary the quanta oi light impressed on said iilm.

5.'A sound recording system comprising means for generating electrical currents corresponding to sound waves to be recorded, a substantially inertialess light producing element, means for positioning said element in a magnetic field of constant strength, an amplifier for said currents,

means for connecting said amplier to said light producing element, a film and an optical system for projecting light from said element to said film, variations oi said electrical currents varying both the intensity and position of said element in said magnetic field to vary the quanta of light impressed on said film, an aperture being provided in said optical system, the width of said aperture controlling the frequency resolution of said light variations with respect to the illumination elllciency of said element.

6. 'I'he method of recording a variable density photographic sound record comprising generating light waves with a substantially inertialess source, projecting said light waves in a predetermined shape to a light sensitive material, and varying the intensity and displacement of said source of light wavesv in` accordance with sound Waves to vary the intensity of said light waves as impressed on said material.

'7. The method in accordance with claim 6 in which said source of light waves have a graduated intensity and are ydisplaced magnetically.

8. The method of recording a variable density photographic sound record comprising generating .a beam of light waves graduated in intensity in two directions, projecting a portion of said light waves to a light sensitive material, and magnetically displacing said generatedbeam in accordance with sound waves to be recorded to project different `portions of said generated beam to said material.

9. The method of recording a variable density photographic sound record comprising generatlng a light beam having a graduated intensity, generating electrical currents corresponding to sound waves to be recorded, projecting a portion of said beam to light sensitive material, and varying the intensity of generation of said light beam to vary the intensity of the portion of said beam projected to said material, said variation in intensity also varying the particular portion of said beam so projected.

10. A sound system comprising a source of light waves, a film, means for projecting said light waves to said film, means for generating a magnetic field, said source being positioned therein, means for generating electrical currents correspending to the instantaneous values of sound waves to be recorded, said currents being impressed on said light source, and means for generating electrical currents for varying said magnetic field to displace said light source.

-' FREDERICK GEORGE ALBIN. 

